A switched mode power supply comprises an input stage for generating an alternating voltage having a varying amplitude and/or frequency from an input voltage, and an output stage for rectifying the alternating voltage and thereby providing a DC voltage to a load. The input stage may comprise a switch that is driven by a pulse width-modulated control signal and that generates a pulse width-modulated alternating voltage. By varying the duty cycle of the control signal voltage regulation of the output voltage can be achieved.
Instead of using a diode rectifier a so-called “synchronous” rectifier (SR) can be used in the output stage. The synchronous rectifier comprises a transistor, e.g., a MOSFET, in particular a power MOSFET, and a control circuit for switching the transistor on and off in a way as to allow a current to flow through the transistor in only one direction. As compared to a diode a MOSFET that is in on state has lower power losses, thus resulting in an increased efficiency of the rectifier circuit.
A power MOSFET has a MOS channel that conducts dependent on a control signal applied to the MOSFET's gate terminal, and has an integrated body diode that conducts, independent of the control signal, when the MOSFET is reverse biased. In conventional switched mode power supplies that comprise a MOSET as a rectifier element the MOSFET is connected such that it is reverse biased in those time periods in which it is expected to conduct for rectifying purposes. In transition periods, i.e., when the transistor is in transition between the forward biased and the reverse biased operating state, and when the MOS channel is not yet conducting the body diode takes over the current through the MOSFET. In general the duration of the time periods when the body diode conducts should be as short as possible in order to minimize power losses.
Switched mode power supplies can be operated in so-called discontinuous current mode (DCM). In DCM there is a time period in each switching cycle of operation when the current through the rectifier circuit is zero before the begin of a following switching cycle. In DCM switched mode power supplies that comprise a synchronous rectifier it is desired to switch the rectifier transistor off when the current through the transistor reaches zero (zero current switching). This requires measuring the current through the transistor, where such current measurement can be performed by measuring a voltage across the transistor. However, when the current approaches zero the voltage also approaches zero, which complicates the voltage measurement during those time periods that are of particular interest. Further, propagation delays in the transistor control circuit make zero current switching difficult. If there is a delay between a time when the transistor control signal assumes a switch-off level and the time when the transistor really switches off, then the switch-off level has to be generated before the current reaches zero, i.e., the switch-off level has to be generated when the current (or the measurement voltage) falls below a switch-off threshold larger than zero. Since propagation delays may be dependent on the temperature adjusting the switch-off threshold is difficult.